With increasing development of high technology industries, computers become essential electronic apparatuses in our daily lives. For example, computers can be employed for work or amusement purposes. For a purpose of performing specialized functions, a computer system may also include one or more peripheral devices linked to the host computer. For example, a scanning apparatus is one of the common computer peripheral devices for scanning images of documents. The scanned images can be converted into electronic files, which are then stored, processed or spread. With the maturity of the scanning technology, the scanning apparatuses are now rapidly gaining in popularity.
Generally, the commercially available scanning apparatuses are classified into two major types, i.e. flatbed scanners and sheet-feeding scanners. As known, the flatbed scanner or the sheet-feeding scanner has a scanning module as a core component. FIG. 1 is a schematic perspective view of a scanning module of a scanning apparatus according to the prior art. As shown in FIG. 1, the scanning module 100 comprises a scanning module case 101, multiple reflective mirrors 102, a lens 103, an optical sensing element 104 and a printed circuit board 106. For clarification, only the components of the scanning module 100 of the scanning apparatus are shown. First of all, a document to be scanned is placed on a glass platform of the scanning apparatus. During operation of the scanning module 100, a light beam is emitted by a light source (not shown) and projected on the document. The light beam reflected by the document is successively reflected by the multiple reflective mirrors 102 and then focused by the lens 103. The focused light is then imaged onto the optical sensing element 104 so as to convert the light beam into an electrical signal.
For accurately projecting the light beam onto the optical sensing element 104 to achieve a sharp image, the relative positions and the relative angles of the components included in the scanning module 100 should be precisely adjusted. Ideally, for each scanning module 100, the sharpest image is obtained when the light beam is focused on a focal point. In addition, depth of focus (or depth of field) is optics concept that measures the tolerance of an optical element. The depth of focus is located within a specified range in front of or behind the focal point. The images within the depth of focus are deemed as sharp images. Due to the depth of focus, the optical element may have an allowable tolerance. For obtaining a sharp image by the scanning apparatus, the optical sensing element 104 needs to be located at the focal point or within the depth-of-focus range.
From the above discussion, it is found that the imaging resolution of the scanning module 100 is determined according to the arrangement of all optical elements included in the scanning module 100. During operation of the scanning module 100, heat is generated from the scanning module 100. If the heat fails to be effectively dissipated away, the temperature of the scanning module 100 will be rapidly increased. Since the scanning module case 101 is made of a plastic material having a high thermal expansion coefficient, the scanning module case 101 will be thermally expanded. As shown in FIG. 1, the optical sensing element 104 is mounted on the printed circuit board 106 and the printed circuit board 106 is directly fastened on the scanning module case 101. Since the optical sensing element 104 is in direct contact with the scanning module case 101, the optical sensing element 104 is compressed by the scanning module case 101 when the scanning module case 101 is thermally expanded. Under this circumstance, the optical sensing element 104 that is originally located at the focal point or within the depth-of-focus range will be shifted such that the optical sensing element 104 is deviated from the focal point or even moved outside the depth-of-focus range. Since the light passing through the lens 103 fails to be focused at the focal point or within the depth-of-focus range, defocus aberration occurs and the imaging resolution of the scanning module 100 is considerably reduced. For correcting the defocus aberration, a heat-dissipating device is required to facilitate removing heat generated by the scanning module so as to maintain normal operation of the scanning module. The use of the heat-dissipating device is neither user-friendly nor ineffective.
Therefore, there is a need of providing a scanning apparatus for preventing defocus aberration so as to obviate the drawbacks encountered from the prior art.